In an effort to block malware, Google has decided it will soon block browser extensions that didn't come from the Chrome Web Store. This change will only affect Windows users (for now), as Google believes the problem doesn't really exist on other platforms.

They elaborate in an official blog post:

Since these malicious extensions are not hosted on the Chrome Web Store, it’s difficult to limit the damage they can cause to our users. As part of our continuing security efforts, we’re announcing a stronger measure to protect Windows users: starting in January on the Windows stable and beta channels, we’ll require all extensions to be hosted in the Chrome Web Store. We’ll continue to support local extension installs during development as well as installs via Enterprise policy, and Chrome Apps will also continue to be supported normally.

While Google officially claims this is a result of security measures, some speculate it's more a move for content control and forcing developers to register for a small $5 fee. We can't know Google's motivation, but it's a little frustrating that the browser can't just offer a warning instead of disabling outside installation for everyone.